Description of Related Art
A typical image sensor senses light by converting impinging photons into electrons that are integrated (collected) in sensor pixels. After completion of integration cycle, charge is converted into a voltage that is supplied to output terminals of the sensor. In CMOS image sensors, the charge to voltage conversion is accomplished directly in the pixels themselves, and the analog pixel voltage is transferred to the output terminals through various pixel addressing and scanning schemes. The pixels have incorporated in them a buffer amplifier, typically a source follower, which drives the sense lines that are connected to the pixels by suitable addressing transistors. After charge to voltage conversion is completed, and the resulting signal transferred out from the pixel array, the pixels are reset in order to be ready for accumulation of new charge. In pixels that are using Floating Diffusion (FD) as the charge detection node, the reset is accomplished by turning on a reset transistor that momentarily conductively connects the FD node to a voltage reference. This step removes collected charge, however, it generates kTC-reset noise as is well known in the art. kTC noise has to be removed from the signal by a complicated Correlated Double Sampling (CDS) signal processing technique in order to achieve a desired low noise performance. For this reason, another class of photo sensing devices and detection nodes have been developed that collect charge in a potential well instead of on a diffusion. These devices are reset by completely transferring collected charge out of the well without leaving any residue. This process thus does not generate kTC noise, therefore, the processing of signals from such pixels is faster and simpler. An example of kTC noise free pixel design can be found in U.S. Pat. No. 6,545,331 B1 to Miida, in U.S. Pat. No. 5,424,223, and U.S. Pat. No. 6,580,106 B2 both to Hynecek. The pixel photo detector described in U.S. Pat. No. 5,424,223 is called the Bulk Charge Modulated Device (BCMD), since collected charge modulates the threshold voltage of a sensing MOS transistor. The advantage of using BCMD devices for charge detection in CMOS image sensors is in their low noise and small size since a single transistor serves there in a charge-to-voltage conversion, the pixel addressing, and signal buffering functions. The disadvantage of using the BCMD devices, on the other hand, is a possibility of interference and cross talk of pixels connected to a common column line. The disadvantage is also the fact that, for an improved performance, the BCMD pixel needs to include a pinned photodiode to achieve high Quantum Efficiency (QE). The pixel also needs to include anti-blooming drain for taking care of charge overflow and charge removal during reset. Adding all of these structures consumes the pixel area and prevents designing of image sensors with very small pixel sizes. Finally, the last disadvantage of the BCMD pixel is in its difficulty to incorporate column reset. The column reset is an important feature for building image sensors with high DR where not all the pixels in a given row are reset at the same time when the row is addressed. In the column reset sensor, architecture the reset signal is supplied to pixels via columns, not via rows. Only a single selected pixel of the array can be reset instead of an entire row.
The present invention addresses these difficulties of conventional BCMD pixel design, and provides a simpler and practical solution where the compact pixel size, the column reset capability, and other performance enhancing features are achieved. This is accomplished by forming a new BCMD device with a Dual Gate structure (DGBCMD).